Increasing the transmission capacity in optical networks requires demultiplexers such as AWG's (also termed waveguide grating router WGR) with wider passband and lower ripple within the passband, particularly since the higher bit-rate signals propagated through the optical networks have wider spectra to be transmitted. Moreover, optical networks using reconfigurable optical add-drop modules (ROADM) have optical signals passing through cascades of demultiplexers and multiplexers. Hence, loss at each stage must be minimized.
AWG “flat-top” designs, where the passband has as broad and flat a shape as possible, have been proposed in the past to widen the passband to expand the transmission characteristics. One example of flat-top AWG's is based on a parabolic horn design, which enables an increase in the 0.5 dB passband to about 50% of the channel spacing, at the expense of some penalty loss.
Other designs have been proposed to improve the passband characteristics with reduced penalty loss, such as in U.S. Pat. No. 5,488,680 by Corrado Dragone in the name of AT&T Corp. As shown in FIG. 1, this prior art design 10 includes a first frequency routing element 12 having at least one input port and P output ports, where P>2 coupled to a grating 14. This approach gives a wide passband with significantly reduced insertion loss compared to the parabolic horn design. In one configuration, Dragone discloses a Mach-Zehnder structure 12 coupled to the input of the AWG 14. The MZI 12 is composed of an input Y-branch coupler 16 which splits the optical power equally in two waveguide arms of different lengths 18, 18′, and a 3 dB coupler 20 at the slab 22 interface of the AWG 14. The MZI 12 disclosed has the same curvature as the second grating 14, meaning that the shorter waveguide 18 of the MZI 12 is on the same side as the shorter arm of the grating One inconvenience of this structure is that it cannot be stacked compactly, which reduces the total number of devices one can print on one wafer.
A similar integrated structure is disclosed in U.S. Pat. No. 6,728,446 by C. R. Doerr in the name of Lucent Technologies Inc. Doerr shows an alternative design 30, seen in FIG. 2, which allows one to stack the MZI-AWG and maximize the number of waveguides. In this layout, the MZI 32 is composed of one Y-branch coupler 34, two waveguide arms of different delays 36, 36′, which are coupled into a full 180 degree coupler 38 which transfers the energy of each arm 36, 36′ into the other arm, and finally a 3 dB coupler 40 at the slab 44 interface of the AWG 42. The 180 degree coupler 38 added in this design allows Doerr to “flip” the curvature of the MZI 32, making the overall layout more compact and stackable. However, the fabrication challenge of realizing a perfect 180 degree coupler that transfers 100% of the light from one arm to the other one across a broad wavelength spectrum is prohibitive. Process variations, wavelength and polarization sensitivity of the coupler can all lead to degraded performance for the overall passband shape in the form of polarization dependent loss (PDL), chromatic dispersion (CD) and polarization mode dispersion (PMD).
A stackable AWG with a broad flat-top passband which overcomes the limitations of the prior art remains highly desirable.